The present invention relates to fire retardant or fire resistant vinylaromatic foams.
Over the past several decades, polymer foams have become available in a wide variety of forms, either in answer to a material need in industry or as a developmental expansion of commercialized precursors. Some of the more popular forms of foamed polymer compositions are shapes such as film, sheet, slab and profiles. Generally, these products are produced by extrusion methods in which the polymer is converted by heat and pressure into a homogeneous melt and forced through a die into the desired shape. To obtain a cellular structure, the plastic usually incorporates a blowing agent that decomposes under the heat of the extrusion process and releases gases that cause the shape to expand. It is also possible to inject propellants directly into the melt.
Because of the favorable combination of properties, price and ease of processing, vinylaromatic polymers, especially polystyrenes, are widely used in preparing foam sheet, film and slab for such divergent end uses as packaging, pipe and tubing, construction and insulation. For example, expanded vinylaromatic polymers such as polystyrene are widely used in the insulation of freezers, coolers, truck bodies, railroad cars, farm buildings, roof decks and residential housing. Vinylaromatic polymer foams are also used as the core material for structural sandwich panels use in refrigerated truck bodies, mobile homes and recreational vehicles.
There is an increasing demand for improving the fire retardant properties of materials used in vehicles, the building trades and consumer goods, such as furniture, radio and television cabinets, appliance housings, electrical motor housings, switch boxes and the like. To meet this demand, various steps have been taken to improve the fire retardant properties of thermoplastic polymers in order to make them slower to ignite and to retard flame propagation. Thermoplastic polymers can be rendered flame retardant by compounding with fire retardant additives, including halogenated organic compounds and inorganic compounds such as antimony oxide. Brominated organic compounds have been used in both foamed and nonfoamed vinylaromatic polymer compositions. It is known to use brominated aliphatic compounds, brominated aromatic compounds and combinations thereof in nonfoamed vinylaromatic polymer compositions. "Fire Resistance Finishing of Thermoplastics as Represented by the Example of Polystyrene," CA87(24):185432e, discloses an unfoamed polystyrene composition containing as flame retardants hexabromocyclododecane, nonabromobiphenyl or decabromodiphenyl ether in combination with antimony oxide "Heat and Flame Resistant Thermoplastic Polymers," CA81(12):64729z, discloses a nonfoamed, impact resistant polystyrene having incorporated therein hexabromobenzene, tribromophenyl allyl ether and hexabromocyclododecane.
Several references have suggested combining hexabromocyclododecane and allyl bromoaromatic compounds as fire retardants in foams. "Fire Resistant Polystyrene Foams," CA97(4):24889a, Japanese Patent 57/38831, Mar. 3, 1982, discloses polystyrene foams comprising polystyrene having incorporated therein a tin stabilizer, talc, hexabromocyclododecane, 2,2-bis-(4-allyloxy-3,5-dibromophenyl) propane, dichlorodifluoromethane and methylene chloride. "Flame Resistant Styrene Polymers," CA95(12):98888e, German Offenlegungsschrift 2,950,098, Jul. 9, 1981; and "Fire Resistant Polystyrene Foams," CA97(8):56865f, Japanese Patent 57/38832, Mar. 3, 1982, also disclose foams containing hexabromocyclododecane and allyl bromoaromatic compounds. The allyl bond is known to be energy absorbant. The presence of the allyl ingredient tends to lower the molecular weight of the polystyrene during processing, thereby degrading its strength characteristics, and/or during the flammability test, in particular, thereby allowing more rapid melting of the polymer away from the ignition source. The allyl bond is unstable in the presence of oxygen and peroxides tend to form which attack and sever the polymer backbone. Thus, regardless of what is said about ranges for this component, actual examples will contain relatively low quantities of the allyl bromoaromatic compounds, i.e., around 0.2 parts per 100 parts of vinylaromatic polymer.
While effective in improving flame retardancy, the incorporation of flame retardant additives into thermoplastic polymer compositions is associated with a variety of problems. Such problems include obtaining homogeneous blending of the matrix resin with the fire retardant additives. Failure to obtain a homogeneous blend can contribute to poor fire retardant ratings. There is a further problem in that fire retardant additives tend to degrade the appearance of the product and can negatively affect physical properties such as impact strength, particularly at higher levels of such additives. In nonfoamed polymers, the bromine content is typically quite high, e.g., in excess of 5 parts of bromine per 100 parts of vinylaromatic polymer.
In foamed vinylaromatic polymers, the degree of bromine loading has to be significantly lower to avoid detrimentally impacting the structural qualities and skin quality of the foam. It is common to use only brominated aliphatics in foamed polystyrenes, with hexabromocyclododecane (HBCD) being the most commonly used foamed vinylaromatic polymer fire retardant. Following current industry practice, the use of brominated aliphatic compounds as fire retardants for thermoplastic polymer foams requires an increase in the fire retardant level as the thickness of the foam increases in order to meet industry test standards such as ASTM E-84, discussed in greater detail below.
The increased level of fire retardant additives results in higher material costs for the product and/or difficulty in maintaining an acceptable appearance of the foam outer surfaces or skins. Skin quality can be preserved by going to a higher foam density, which is accompanied by an increase in raw material costs. There is a need for more efficient fire retardant systems for use with thermoplastic vinylaromatic polymer compositions. Additionally, because conventional blowing agents, particularly physical blowing agents such as halogenated hydrocarbons, are considered to be environmentally detrimental, there is a need for flame retardant systems which can function with minimal corrosive effects in pristine expansion systems in which water is at least a component.